A decline in support for GOP Gov. Scott Walker among independents and college-educated voters is making the 2018 election a much tougher struggle for Walker than his three previous statewide races, according to findings of an exit poll of Wisconsin voters Tuesday night.

Walker had won independent voters in 2010, the 2012 recall and 2014 by margins ranging from nine to 14 points, according to exit polls in those contests.

But in the latest 2018 exit poll numbers, the governor is losing independents. Overall, the exit poll numbers are too close to indicate who might win the race between Walker and Democrat Tony Evers.

In the U.S. Senate race, Democrat Tammy Baldwin defeated GOP opponent Leah Vukmir across most demographic groups and was declared the winner by the networks shortly after the polls closed.

The exit polls results reported in this story are based on a completed survey of voters conducted at the polling places Tuesday, supplemented by a telephone poll of people who voted before Election Day.

The survey is conducted by Edison Research, under a cooperative arrangement among ABC, CBS, NBC and CNN.

The exit poll numbers reported here, after the polls have closed in Wisconsin, are not final: they will undergo some changes later in the evening, as they are adjusted to the actual returns. But they shed broad light on patterns of support — and shifts from past years — among different segments of the electorate.

A second key group that appears to be supporting Walker at lower levels is college-educated voters.

In 2014, Walker won college grads by 1 point, but the 2018 exit poll is showing Walker losing that group by double-digits.

The governor is winning “non-college” voters as he did in previous races.

But the “education gap” — the difference between voters with college degrees and voters without college degrees — is much bigger in this race than in Walker’s past elections, in keeping with one of the signatures of the Trump era, a growing political divide based on education levels.

In the U.S. Senate race, Baldwin led Vukmir in the exit poll among both college and “non-college” voters, but her margin is much larger among voters with a college education.

Baldwin led Vukmir by especially large margins among women, independents, and moderates. She led 4-to-1 among voters who said health care was the most important issue facing the country — and those voters represented about half the electorate.

Baldwin was doing far better among rural and small-town voters – a GOP-leaning segment of the electorate – than the Democrats’ losing US Senate candidate two years ago, Russ Feingold, according to the exit poll.

More than two-thirds of Wisconsin voters think the economy is good or excellent, according to the exit poll.

But more disapprove (54%) than approve (45%) of President Donald Trump.

Trump’s approval rating in the early exit poll data is slightly lower than it was in the final pre-election poll of likely voters in Wisconsin by the Marquette University Law School (47 percent).

Offered four choices, about half of all voters said health care is the most important issue facing the country, followed by the economy (about one in four), immigration (about one in five) and gun policy (under 10 percent).

Voters who chose health care as the most important issue were more likely to vote Democratic and those who chose the economy or immigration were more likely to vote Republican.